LMDs are androids that mimmik a person and have long been used as doubles by SHIELD, Nick Fury and Doctor Doom in the comics. If this were any other show that would be that, but this is a Marvel show about SHIELD and SHIELD has a technology called the LMDs. LMD stands for Life Model Decoy it is basically an android that perfectly resembles a person. Normally, LMDs are used to hide the actual person's movements. The biggest user of the LMD technology in the Marvel Universe is Nick Fury. In the Marvel Cinematic Universe LMDs have already been mentioned by Tony Stark in the Avengers so it known that they exist.

Agents of SHIELD has been playing with the concept of LMDs since season 1 when they introduced Eric Koenig's seemingly identical brother Billy is introduced. Eric dies in season 1 but yet another Koenig brother Sam is introduced in Season 2. Theories have continued to grow whether the Koenigs are an original LMD, or clones or what.

The reveal of what the Koenigs are occurs in Season 4 when it is revealed that the Koenig brothers were Quadruplets who along with their sister all (Except for Thurston) joined SHIELD and became close confidants of Nick Fury.

Eric Koenig - First Koenig introduced in season 1 was killed by Ward in Season 1

Billy Koenig- Introduced in Season 1 after Eric had been killed

Sam Koenig - Introduced in Season 2

Thurston Koenig - Black Sheep in the family as he did not join SHIELD like the others.

LT Koenig - The non quadruplet sister of the Koenig brothers. She was the first Koenig to join SHIELD

The Koenig brothers have all been played by Patton Oswalt while LT Koenig was played by actress Artemis Pebdani.

Original meeting of Eric Koenig
Hydra attacks our favorite agents are on the run and Coulson finds a coordinates to a super secret SHIELD base called Providence. At Providence they meet quirky Agent Eric Koenig (Played by Patton Oswalt).

Hi I'm Eric Koenig

Agent Koenig is a likable but fairly distrustful fellow. He is also a close confidant of Nick Fury. It is Koenig who tells Coulson that Nick Fury is still alive. It also Koenig who reveal Agent Triplett had a grandfather in the howling commandos. Koenig then conducts a lie detector test on the agents trying to sniff out Hydra sleepers. Unfortunately, Ward (A hydra sleeper) manages to outsmart the test and remains hidden from the group. Later Ward kills Eric Koenig.

At one point Eric Koenig asks if Coulson would want to play video game with Koenig's brother. Apparently, the two could play online through the base. This is the only reference to Koenig's brother that I am aware of.

At the end of the season the team once again finds another super secret SHIELD base called the "Playground." At the Playground the team is met by the bases caretaker who looks and acts exactly like Eric Koenig. He says his name is Billy Koenig, Eric's twin brother. This is where the LMD speculation effectively began.